In nuclear well logging, it is frequently necessary to determine the energy that a particle or photon has deposited in a detection device. These detection devices provide an electrical signal that is indicative of the amount of energy deposited in a single event. The energy distribution of the gamma rays from a multitude of elements can be represented as a histogram, in which the abscissa represents the deposited energy or a function thereof, and the ordinate the number of events having a signal which falls into one of the discrete bins of the abscissa.
There are many variants of nuclear detectors which are well known in the field of nuclear measurements. A nuclear detector typically consists of a detecting material and a device or devices to convert and/or amplify the signal produced by the detector. Such detectors can include semiconductor detectors such as Germanium-detectors or scintillation detectors coupled to photon detectors, proportional counters, or the like.
The purpose of a gamma ray spectroscopy system is to determine the energy associated with the absorption of incident gamma rays by the detector (pulse events). Pulse events can be registered in histograms organized by energy levels (Multi-channel Analyzer, or MCA, spectra) and/or times of arrival (Multi-Channel Scaler, or MCS, spectra). The performance of such systems is measured in terms of energy resolution (ability to distinguish between two separate but adjacent energy levels), time resolution (ability to distinguish between two nearly coincident pulses), throughput (ability to process multiple adjacent pulses) and linearity (linear relationship between deposited energy in a pulse and associated histogram channel). A typical gamma ray acquisition system 100 is shown in FIG. 1 and includes detector 102, preamplifier 104, pulse height/MCS analyzer 108 and histogram acquisition memory 110. Detector signals associated with pulse events are typically fast rising unipolar pulses (rise time <10 ns) with a slower exponential decay (single or multiple time constants). Fall time constant(s) vary from 1 ns to a few μs, depending on the type of detector. The system 100 may additionally include well-known components such as a display 112, a microprocessor 114 and data storage/memory 116.
Classic pulse height analyzers rely on a shaping amplifier driving a sample-and-hold circuit (pulse stretcher) connected to an analog-to-digital convertor (ADC). The pulse shaper is a dispersive filter (pseudo Gaussian impulse response filters are typically used). Its purpose is two-fold: [1] increase the detector signal rise time to make it more suitable for peak sampling and [2] speed up signal return to baseline (by eliminating asymptotic behavior) to improve time resolution and reduce inter-symbol dependency. Such analyzers measure shaped pulse peak amplitudes and provide for good energy resolution (since the signal to digitize is being held constant by the sample & hold circuit). However, they have a limited throughput, due to ADC conversion time (typically 5 to 10 μs) and operate in a discontinuous fashion (i.e., the analyzer is disabled during a conversion). Such performance limitations make them inadequate to process the high count rates, which can be acquired by modern detectors.